The present invention relates to measurement of corrosive characteristics of a fluid and more particularly concerns a corrosion measuring probe that can be mounted flush with the interior of a fluid pipe or container.
A common method of continuous measurement of corrosive characteristics of a fluid employs resistance measurement of a metallic, corrodible test element to indicate, by change of resistance, the amount of metal that has been lost by corrosion over a period of time. A widely used instrument for this measurement is known as a Corrosometer probe manufactured by Rohrback Corporation, assignee of the present application. A probe of this type is described in U.S. Pat. No. 4,338,563 for Corrosion Measurement With Secondary Temperature Compensation, issued to Rex V. Rhoades and James L. Geer. In one form, the probe employs a long, tubular, metallic test element carrying an inner reference element made of the same material as the test element. The interior of the tubular test element is filled with a thermally conductive, electrically nonconductive compound. Alternating current is passed through the elements, and electrical resistance of each is measured while or after the probe has been immersed in an environment of which corrosive tendencies are to be monitored. Because electrical resistance of the metal changes with the amount of metal in the test element, measurement of test element resistance provides an indication of corrosion. Because electrical resistance of the metal also changes with temperature, a reference element is provided made of the same material as the test element and having the same temperature resistance characteristic. Therefore, changes in resistance of the test element that are due to long term temperature variation are eliminated by comparison of resistances of the test and reference elements.
Long tubular probes of the type shown in U.S. Pat. No. 4,338,563 are generally used by immersion in the fluid of which corrosive tendencies are to be sensed, and the entire exterior surface of a part of the tubular probe acts as the test element. Such a probe is not nearly as suitable for measurement of fluid flowing in pipe, and, in such applications, it is preferred to use a probe having a sensing or test element that is substantially flush with the interior surface of the pipe. A flush probe will minimize disturbance to fluid flow caused by the measurement and will provide more reliable corrosion measurement. One type of such flush probe includes a probe body that extends through a pipe wall and has an end that is flush with the pipe interior. The end of the body is filled with a nonconductive insulating and sealing glass, and a metallic ribbon of a test material is mounted on the end of the glass and has electrically conductive end portions extending through the glass end seal to the interior of the probe body. Such a probe is useful only with a small number of special metals, if the probe must be employed over a wide temperature range as is commonly required. In pipelines and chemical plants, corrosive fluid temperatures may be in the order of 400.degree. F. to 500.degree. F. At such temperatures the great difference in the coefficient of linear expansion between the metal of the test ribbon and the glass of the seal frequently breaks the seal and thus prevents any extended life for such a probe. Glass seal flush probes are not sufficiently reliable nor rugged enough, and at least partly for this reason, flush probes are not as widely used as they might be. If more reliable and rugged flush probes were available, their use would be significantly increased.
Accordingly, it is an object of the present invention to provide a flush corrosion resistance probe that avoids or eliminates above-mentioned problems.